It is frequently necessary to provide a seal between two rotating components, for example internal shafts and rotors of gas turbine engines. It is well known to use labyrinth seals in such applications. However, labyrinth seals allow significant leakage, which gets worse with time.
More recently, air-riding seals have been developed, for example as disclosed in US2010/0213674. Such a seal comprises a sealing ring, for example of carbon, which is mounted on one of the components so as to be rotationally fixed but axially displaceable. The sealing ring rotates next to a runner mounted on the other of the components. In operation, there is a small axial gap between confronting radial surfaces of the sealing ring and the runner, and one or both of the radial surfaces is profiled, such that, when relative rotation occurs between the components, aerodynamic lift is generated by the film of air in the axial gap between the radial surfaces to prevent them from coming into contact with one another.
Air-riding seals are designed in such a way that the width of the axial gaps is not affected by centrifugal effects or thermal growth. In the absence of contact between the sealing ring and the runner, wear is eliminated except at very low relative speeds. In a previously proposed sealing arrangement, a carbon sealing ring is disposed between two profiled runners so that an air-riding effect is achieved on both sides of the carbon ring. This causes the carbon ring to be centralised between the runners, maintaining a good seal. The carbon ring is a split ring and is therefore radially resilient to enable it to maintain contact with the component on which it is mounted. The split in the ring provides a route for leakage across the seal and in addition some leakage can occur through the air-riding gap between the carbon ring and the runners on each side. Apart from the loss of efficiency which can result from such leakage, in some applications hot gas can leak past the seal into an oil environment, creating a fire risk. Alternatively, oil leaking past the seal can result in oil loss.